1. Field of the Invention
The present invention relates to a cathode ray tube (CRT) and more particularly, to a cathode ray tube capable of reducing the deflection power consumption.
2. Description of the Related Art
A CRT is a device for displaying image on a screen by vertically and horizontally deflecting electron beams generated from an electron gun and landing the deflected electron beams onto the phosphor layers formed on the screen. The deflection of the electron beam is controlled by a deflection yoke, which is mounted on the exterior surface of the funnel of the CRT, and forms vertical and horizontal magnetic fields. The CRTs are generally employed for color televisions (TVs), monitors and high definition televisions(HDTVs). With increasing use of the CRTs, there is a need for reducing the length of the CRT to increase the brightness of the displayed image and to reduce the size of the final products, such as TVs, monitors and HDTVs.
When reducing the length of the CRT, the electron beams should be deflected with wide-angles, and the deflection frequency and current supplied to the deflection yoke should be increased for the wide-angle deflections of the electron beams. As the deflection frequency and current increase, the deflection magnetic field tends to leak to the outside of the cathode ray tube and the power consumption increases.
In order to decrease deflection power and magnetic field leakage at the same time, it is conventionally preferable to decrease the neck diameter of the cathode ray tube and the outer diameter of the funnel near the neck on which the deflection yoke is mounted, so that the deflection field efficiently acts on the electron beams. However, when the neck diameter is simply decreased, there are some disadvantages including deterioration of the image resolution due to the reduced diameter of the electron gun, and likely bombardment of the inner wall of the funnel by the outer electron beams, resulting in the electron beams not properly landing on the phosphor layer of the screen.
In order to solve these problems, U.S. Pat. No. 3,731,129 discloses a funnel having a wider peripheral portion sealed to the periphery of the panel, and a deflection portion whose cross-sectional configuration gradually varies from a rectangular shape substantially similar to that of the rectangular image produced on the panel to a circular shape. Thereby, the vertical and horizontal coils of the deflection yoke are closely located to the passage of the electron beams, and deflect the electron beams with reduced deflection power and without bombarding the electron beams to the inner wall of the funnel. However, the funnel does not have enough strength to endure against external stress, such as pressure, thus the funnel has to be designed to have a circular or round cross section.
Meanwhile Japanese Laid Open Patent 9-306388 corresponding to U.S. Pat. No. 5,763,995 discloses a funnel, whose cross section of the exterior surface near the neck changes from a circular shape to a non-circular shape with a maximum diameter along a direction (diagonal direction) other than the horizontal direction or the vertical direction. In addition, the funnel is designed to meet the following condition. EQU 0.3&lt;.DELTA.HV/L.ltoreq.0.6
where L is the length of the maximum diameter, and .DELTA.HV is the sum of .DELTA.H and .DELTA.V, and .DELTA.H is a difference between L and the horizontal diameter of the cross-section of the funnel, and .DELTA.V is the difference between L and the vertical diameter of the cross-section of the funnel. However, the funnel is defined or configured by three variables, .DELTA.H, .DELTA.V and L. Thus, for example, even though .DELTA.H is set to a fixed value, two variables are not fixed or defined. As a result, it is difficult to design a funnel having the optimum configuration and enough strength against external stress. PA1 where SA is the vertical diameter of the external surface of the funnel, and LA is the horizontal diameter of the external surface of the funnel, and DA is the maximum diameter of the external surface of the funnel. Thus, the funnel is also defined by three variables, SA, LA and DA, and it is also difficult to design a funnel having the optimum configuration and enough strength against external stress. PA1 where rh is the diameter of the funnel in the direction of the long axis of the panel, and rv is the diameter of the funnel in the direction of the short axis of the panel.
Meanwhile, Japanese Laid Open Patent 10-149785 discloses a funnel whose cross section of the exterior surface is a non-circular shape with a maximum diameter along a direction (diagonal direction) other than the horizontal direction or the vertical direction. In addition, when the aspect ratio of the screen is M:N, the cross section of the funnel is designed to meet the following condition. EQU (M+N)/(2(M.sup.2 +N.sup.2)1/2)&lt;(SA+LA)/(2DA).ltoreq.0.86